A bar dip is a piece of fitness equipment that consists of a U-shaped bar, usually about 1" (2.5 cm) in diameter, which surrounds the user's body at the waist. The user grips the bar with both hands, and supports their bodyweight on it. The exercise is often performed in conjunction with the user wearing a dip belt and additional weight to increase resistance.
The exercise involves raising and lowering the body with the triceps, chest, rhomboid, and deltoid muscles. By leaning forward the user can work the chest more and the triceps less. Conversely, by keeping the body upright, the triceps are worked to a greater extent than the chest.
An image of a bar dip is here: http://www.frixo.com/sites/fitness/exercises/dips.html